This invention relates to drive units, and in particular, to electrical drives having soft starting characteristics. While the invention is described in particular detail with respect to its application as a drive unit for post office conveyors, those skilled in the art will recognize the wider applicability of the inventive principals and structure disclosed hereinafter.
In this specification, the term "soft start drive" is used to indicate a drive system for a driven unit which enables the driven unit to come up to operating speed at a rate slower than the rate at which the driving member, conventionally an electric motor, comes to speed. Such drives find application, for example, in conveyor systems, in large blower units and other apparatus having high inertia during start-up.
The particular factual background from which the present invention was derived deals with conveyor drives. It is common, in many conveyor applications, to use constant speed motors, of 1/2 horsepower and larger, to drive the conveyor system. Constant speed motors generally are less costly to purchase than variable speed motors, also known in the art, particularly if additional low cost soft start mechanisms can be used with them. Thus, various post office installations have opted to utilize electrically powered conveyors driven by constant speed motors for carrying the mail through the post office facility during processing. This type of application has a varying load factor in that the amount of mail processed in certain periods of the work year, for example, during the Christmas season, is substantially higher than other periods. In general, the post office applications require motor systems for driving the conveyor which are powerful enough to handle the peak load condition. This means the drive system is too powerful for a large portion of the remaining operational time periods of the conveyor.
To overcome these difficulties, it heretofore has been the practice to provide a dry fluid drive between the motor and a gear system which is connected to the conveyor. The dry fluid drive conventionally included a chamber which contains a plurality of steel shot. A system illustrating this form of dry fluid drive is shown in FIG. 5 of the drawings, and has been labeled prior art. It may be observed, in FIG. 5, that a plurality of vanes are arranged in a chamber defined by the enclosure of the dry fluid drive. The vanes function to engage the shot load and compress it as the motor begins to come to speed. The time required to compress the shot load enables the conveyor to accelerate at a slower rate than the drive motor, and supposedly provides the desired soft start characteristic for the system.
While dry fluid drives work well for their intended purposes in theory, they have exhibited a number of deficiencies in applicational use. First, the shot load in this type of drive must be adjusted to compensate for conveyor loading. Because most conveyors operate against a variable load factor, it is impossible to match correctly the dry fluid drive to the conveyor for the continually changing nature of the load. Consequently, under many loading conditions of the conveyor, the soft start characteristics of the system are lost. Second, maintenance of the dry fluid drive is troublesome. Because steel shot is used, environmental conditions can affect the shot adversely. Rusting is a particularly troublesome and recurrent problem. Rusting in turn permits the shot to powder over a period of time. These factors, among others, have meant that dry fluid drives require high maintenance expenditures, in both time and money. Postal applications require a drive that is adjustable to compensate for various load conditions, thereby providing the desired soft start characteristics, but that is unencumbered by the dry fluid drive deficiencies.
Electromechanical clutches also are well known in the art. In general, these devices include a first and second member which are separated from one another in the disengaged or off position of the clutch. Upon activation, electrical energy is applied to the field of the electromechanical clutch which generates a force tending to draw the second member against the first member to initiate clutch engagement. It has been common practice to adjust the voltage applied to the field of the electromechanical clutch in order to match the clutch torque to the load.
The invention disclosed hereinafter, while utilizing an electromechanical clutch as part of the overall system, differs from prior art clutch designs in a number of material aspects. As later described, the first and second members of the clutch are continuously engaged, regardless of whether or not electrical energy is supplied to the field winding. This condition is known in the art, and is defined for the purposes of this specification, as zero air gap. The facing material of one of the first and second clutch members which, as indicated, always are in contact with one another, is a low coefficient of friction material, which permits relative rotation of the first and second clutch members. Electrical control means is provided for controlling the application of electrical energy to the clutch. Initial voltage applied to the clutch is adjustable, so that the accelerating torque of the system can be adjusted to match the load. The circuit further, however, automatically applies a second, higher voltage to the clutch so that the clutch field is energized first by an accelerating voltage and subsequently by a running voltage. Running voltage for the clutch is chosen so that the breakdown torque of the clutch is higher than the breakdown torque of the associated dynamoelectric drive means, for example, a three phase electric motor. Such motors generally have overload protection means associated with them. Because clutch breakdown torque is higher than motor breakdown torque, the overload protection means for the motor will trip before the clutch of my invention overloads.
One of the objects of this invention is to provide an improved soft start drive system.
Another object of this invention is to provide a soft start drive system including a clutch having zero air gap between the rotating members of the clutch in all positions of the rotating members.
Yet another object of this invention is to provide a clutch, at least one member of which includes a face constructed from a low coefficient of friction material which permits relative rotation of the clutch members during the system start-up.
Another object of this invention is to provide control means for applying electrical energy to an electromechanical clutch of a drive system which includes means for supplying an adjustable first voltage to the field of the clutch for permitting adjustment of the soft start characteristics of drive system to a variety of load conditions.
Yet another object of this invention is to provide control means for applying electrical energy to an electromechanical clutch of a drive system which includes means for supplying a second voltage to the field of the clutch after the application of a first voltage, the second voltage ensuring that the breakdown torque of the clutch will be higher than the breakdown torque of any associated dynamoelectric drive means.
Another object of this invention is to provide an electric circuit means for controlling application of voltage to an electromechanical clutch of a drive system, which circuit provides first and second voltages to the field of the clutch in accordance with a sensed condition.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.